1. Field of the Invention
This invention relates to fibrous sheet material having improved bulk and strength characteristics and a method and apparatus for manufacturing it.
2. Description of Related Art
Many products made of fibrous sheet material, such as paper, require softness, absorbancy and strength. A principal characteristic of such material, contributing to absorbancy and subjective softness, is the bulk or compressability of the material. Conventional manufacturing techniques produce high bulk paper products by avoiding significant compression of the wet fibrous web during the manufacturing process and by creping or embossing the fibrous web and/or adding chemical debonders. These known processes achieve desired bulk by minimizing inter-fiber bonding. Minimizing compression of the web reduces interfiber contact and, therefore, reduces bonding. Creping achieves this result by breaking a substantial fraction of the fiber bonds. Embossing achieves the result by mechanically pressing the web onto a open-mesh imprinting fabric, or other patterned surface, such that a substantial portion of the web area is impressed into voids and the web is subjected to significant compaction only at the non-void areas thus confining the fiber bonding to the non-void areas. Chemical debonders, which may be used with any of the other known techniques, further reduce interfiber bonding.
The strength of the paper is achieved through fiber bonding. Known high-bulk fibrous material, therefore, generally has low strength. In known processes, the strength of the paper product must be sacrificed in order to achieve the high-bulk desired for softness and absorbancy.
Additionally, the known processes which avoid compression of the wet web before embossing are energy intensive. It generally requires less energy to remove water from a web through compression than with heat. By avoiding compression, substantially all drying of the web must be performed with heating techniques.
Examples of recent efforts to achieve soft, absorbant fiber sheet material having commercially acceptable strength characteristics are represented by U.S. Pat. Nos. 3,821,068; 4,208,459; 4,309,246; 4,356,059; 4,420,372; 4,421,600; 4,440,597; 4,429,480; 4,551,199 and Re28459. In each of these patents, a method of manufacturing a high-bulk paper products is described which seeks to maximize bulk while minimizing the loss of strength. Each of these known methods is energy intensive and produces a high bulk product of lower strength than provided by this invention.
The present invention improves upon known apparatus and methods of manufacturing high-bulk products resulting in a paper product having significant bulk and substantially improved strength characteristics. In particular, at the same basis weight, the product of the invention compares to conventionally creped and uncreped products as follows:
______________________________________ Characteristic Conv. crepe Conv. uncrepe ______________________________________ caliper &gt; &gt; apparent bulk &gt; &gt; tensile strength &gt; &lt; air permeability .ltoreq. &gt; oil holding capacity .ltoreq. &gt; machine direction (MD) softness &gt; &lt; cross direction (CD) softness &gt; &gt; MD total energy absorb. &gt; &lt; CD total energy absorb. &gt; &lt; ______________________________________